Inactivation of defective interfering (DI) particles of vesicular stomatitis virus (VSV) by ultraviolet light was measured. The data followed single hit kinetics, regardless of the size of the RNA of a particular DI particle. The results suggested that interference by DI particles requires a conservation of intact RNA terminal sequences, both at the 3' and 5' ends. The exceptional HR DI particle did not follow this rule. In this case the target size for interference corresponded to 43% of the HR DI particle RNA. This suggested two possible mechanisms for interference by VSV DI particles. HR DI particle is unique in its genomic content and in its ability to transcribe and translate its genetic information. The possibility that this particle might interfere with viral infection on the level of primary transcription will be investgated. In order to distinguish between transcripts from the virion and transcripts from the HR DI particle genome, the virion of the New Jersey serotype will be utilized (HR DI particles interfere equally efficiently with both serotypes). The transcripts will be assayed by annealing with virion RNA. Since the New Jersey serotype RNA does not anneal to the HR Indiana transcripts the assay will specifically register virion transcripts. Several DI particles of the Chandipura, Mokola and Kern Canyon rhabdoviruses were isolated and their RNA characterized. Some of these particles were found to contain homogeneous RNA species. It is planned to map these RNAs by annealing with the fractionated individual mRNA species. The purpose of these experiments is to ascertain whether any general genetic locus for interference exists in all rhabdoviruses.